The present invention relates to a multilayer thin film and a sputtering deposition method thereof. In particular, the present invention relates to a method of depositing a multilayer thin film by means of magnetron sputtering which controls the magnetic field.
At present, a multilayer film is formed by depositing it with an evaporation or a sputtering method using two or more different materials. When two or more materials are deposited, two or more sputtering cathodes are needed (as taught by, for example, Masaki Koike et al. xe2x80x9cNanofabrication of Multilayer Zone Plates by Helicon Plasma Sputteringxe2x80x9d Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 6754-6757). The reference discloses a technique of forming multilayer zone plates by depositing alternately Ag layers and Al layers with the sputtering technique which employs two helicon cathodes. In addition, a one cathode technique using composite targets is proposed (as taught by, for example, Brij. B. Lai et al. xe2x80x9cMagnetic and recording properties of monolayer and multilayer thin-film media by using composite targets. J. Appl. Phys. 79(8), Apr. 15, 1995, pp. 5336-5338). The reference discloses a method of forming a magnetic film with Cr as an underlayer using concentric annular shaped composite targets of Cr and magnetic materials.
The use of two or more cathodes, however, makes a sputtering system complex and makes it difficult to control thicknesses of respective layers and impurity contents. That is, if two materials are deposited simultaneously, compositions within a substrate become uneven due to separation in the space. Meanwhile, if alternate deposition is conducted between two materials, interfacial films of quite low nanometer level are disadvantageously formed among the layers due to switching of deposition modes.
It is meanwhile well known that the sputtering system is a system for depositing layers of certain materials on a substrate. This includes a magnetron sputtering system utilizing a magnetic field for purposes of accelerating the sputtering rate of deposited materials. In the system, the application of a magnetic field crossing an electrical field causes electrons emitted from a cathode to make a trochoid movement and high density plasma is generated on a target, thus making it possible to increase sputtering rate with relatively low voltage.
FIG. 1 is a typical magnetron sputtering system. It has a cathode section 13 and an anode 15 within a vacuum chamber 11. A to-be-processed substrate 17 is provided at the anode 15. The cathode section 13 has a plurality of magnets 19. A material 21 referred to as a target hereinafter is mounted on a susceptor 23 of the cathode section 13. In recent years, a system wherein an anode 15 and a cathode section 13 are turned upside down has been frequently used. This is because it is preferable for carrying a to-be-processed substrate.
If the system is operated, a container 11 is evacuated from an exhaust outlet and inert gas, such as argon, is injected from an injection port at low pressure. DC or RF power is applied onto the cathode section 13. The magnets 19 form a closed magnetic circuit of strong magnetic field on the surface of the target 21. If such a magnetic field exists on the target 21, then electrons make a trochoid movement and enclosed in the vicinity of the target 21 and the electrons and gas molecules collide with each other more frequently.
The inert gas within the container 11 collides with accelerated electrons and turns into ions. As a result, plasma occurs in the vicinity of the cathode section 13. Positive gas ions from the plasma, which are accelerated in the cathode section direction, collide with the target 21 and expel some of target materials out of the target 21. The expelled materials are deposited on the substrate 17.
In this way, the magnetic field increases electron density on the target 21, thereby increasing ionization ratio in this region. Although the magnetic field has been long used for improving the degree of sputtering deposition, it has been aimed only to increase the deposition rate of materials.
The magnetron sputtering system now on the market employs a fixed magnetic field or an alternating magnetic field. The fixed magnetic field is realized by installing permanent magnets to cover the entire back surface of a target as shown in the above description. It is also realized by using electromagnets through which fixed magnetizing current flows. The alternating magnetic field can be realized by moving periodically permanent magnet pieces to the back surface of the target or by using electromagnets with which a single or a plurality of coils are magnetized by periodically changing current.
As described above, sputtering rate can be increased by using the magnetic field. A method of controlling thicknesses of the layers of the multilayer film and impurity concentration with high accuracy has not been however known.
It is therefore the first object of the present invention to provide a method of depositing a multilayer film, which method is capable of accurately controlling film thickness.
It is the second object of the present invention to provide a method of depositing a multilayer film, which method is capable of accurately controlling composition.
To attain the above objects, a thin film deposition method in the first aspect of the present invention comprises the steps of:
preparing a magnetron sputtering system having magnetic field generation means for changing a magnetic field;
mounting, as a target, a composite material including not less than two components on a cathode of the magnetron sputtering system;
providing a to-be-processed substrate on an anode of the magnetron sputtering system;
evacuating a chamber of the magnetron sputtering system and thereafter filling the chamber with inert gas; and
controlling a cycle of the alternating magnetic field to change a ratio of the not less than two components of the thin film in a film thickness direction of the thin film, by applying, onto the cathode, one of DC power and RF power and, at the same time, the alternating magnetic field from a lower portion of the target.
The magnetic field generation means has an electromagnet, and the step of controlling a cycle of the alternating magnetic field can include a step of changing a cycle of magnetizing current of the electromagnet.
The magnetic field generation means has a permanent magnet, and the step of controlling a cycle of the alternating magnetic field can include a step of controlling a cycle of moving the permanent magnet below the target.
It is preferable that the composite material serving as the target is a composite metal including not less than two metal elements.
The composite metal is preferably one selected from a group consisting of WSi, CoFe, CoCu, CoCr, FeCu, FeNi, MnNi, ternary combinations of three components, CoCrTa and FeNiCoMnCu, the ternary combinations of three components including three selected from a group consisting of Co, Fe, Cu, Cr, Ni and Mn.
A multilayer thin film deposition method in the second aspect of the present invention comprises the steps of:
preparing a magnetron sputtering system having magnetic field generation means for changing a magnetic field;
mounting, as a target, a composite material including not less than two components on a cathode of the magnetron sputtering system;
providing a to-be-processed substrate on an anode of the magnetron sputtering system;
evacuating a chamber of the magnetron sputtering system and thereafter filling the chamber with inert gas; and
applying, onto the cathode, one of DC power and RF power and, at the same time, an alternating magnetic field having a predetermined maximum value from a lower portion of the target to obtain a predetermined composition changing in a film thickness direction of the thin film.
The magnetic field generation means has an electromagnet, and the step of applying the alternating magnetic field can include a step of changing a maximum value of the alternating magnetic field by changing magnetizing current of the electromagnet.
It is preferable that the composite material serving as the target is a composite metal including not less than two metal elements.
The composite metal is preferably one selected from a group consisting of WSi, CoFe, CoCu, CoCr, FeCu, FeNi, MnNi, ternary combinations of three components, CoCrTa and FeNiCoMnCu, the ternary combinations of three components being selected from a group consisting of Co, Fe, Cu, Cr, Ni and Mn.
Moreover, a thin film according to the present invention includes at least two components, a composition of the components periodically changing in a film thickness direction, concentrations of the at least two components gradually changing in a common cycle in a film thickness direction of the thin film, the cycle of one of the at least two components is shifted by a predetermined rate in the film thickness direction from the cycle of the other of the at least two components.
The thin film can be formed by sweeping a magnetic field while using different sputtering directivity of the at least two components in magnetron sputtering.
The thin film can further comprise a substrate on which the thin film is formed.
The at least two components are made of metal and constitute a composite metal.
It is preferable that the composite metal is one selected from a group consisting of WSi, CoFe, CoCu, CoCr, FeCu, FeNi, MnNi, ternary combinations of three components, CoCrTa and FeNiCoMnCu, the ternary combinations of three components including three selected from a group consisting of Co, Fe, Cu, Cr, Ni and Mn.
A distance between peaks of the concentration of one of the at least two components is not more than 100 nm.
According to the present invention, the magnetic field is cycled to enhance deposition rate of one material over the other. A composite targets consisting of two or more materials is employed and parameters of the magnetic field are changed. By doing so, the relative material sputtering rate is increased and therefore a finely controlled multilayer film can be deposited.
The first aspect of the present invention is to change magnetic field sweep time and thicknesses of deposited layers. The deposited layer thickness is inversely proportional to the magnetic field sweep time.
The second aspect of the present invention is to change the relative composition of sputter species by changing magnetic force. With electromagnets, magnetic force is changed by adjusting the amplitude of input current waveform, i.e., maximum current.
Additional object and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore.